The use of sys.platform is very risky, because it is
not at all clear what the possible values are.  For
example, on Mac OS X "darwin" is used, but this can
only be discovered by trying it out and asking Jack
when another value will be used.

Another problem is that the examples given include a
number, which suggests that an OS version number might
be included here.  Is that only a major version number?
 And why doesn't Mac OS X report "macosx10.2"?

Suggestion: Add a list of known values and for what
platforms they are used.

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A first step would be to document that for Unix the value is based 
on the information returned by "uname" *at the time of building 
Python*, mapped to lowercase and with some cruft removed.

For other platforms for which the value is known (Windows, 
MacOS9) we could document the actual values.

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How often are new platforms supported?  From my short time on 
python-dev I would say not very often.  So listing all known 
platforms shouldn't suffer *too* much from bit rot, but it is 
something to consider before going forward with this.

Implemented the suggestions in r60136.